| According to the research and development of semisolid process at present, in this paper, a novel process, Vibrating Wavelike Sloping Plate process-VWSP, for preparing semi-solid alloy was proposed, according to the Natural Science Foundation of China, Al-6Si-2Mg alloy was selected as study object, heat and solute diffusion, solidification thermodynamics and kinetics during dynamical solidification process under action of vibrating wavelike sloping plate were studied. Fine non-dendrite microstructure formation mechanism and effects of process conditions on the microstructure of semi-solid alloy were studied, and the process conditions were optimized. The reheating and thixoforming of semisolid billet prepared by VWSP process were studied. Main work and results were described as follows:(1) Semi-solid alloy material was successfully prepared by using a self-designed vibrating wavelike sloping plate process device. When alloy melt was cast on the wavelike sloping plate, under actions of vibration and flow, alloy nucleated and growed, which finally led to fine and spherical microstructure formation.(2) Through numerical simulation and experiment, temperature distributions of alloy on the sloping plate with the length of 400mm and the sloping angle of 45°were obtained. The decreasing value of temperature from the casting mouth to the exit was in the scope of 66℃～77℃. The decreasing velocity on the melt bottom was fastest, that on the side wall of melt was secondly faster, that on the melt surface was relatively slowly, and that on the symmetrical plane of the width was most slowly. Vibrating wavelike sloping plate could more easily kept even temperature distribution of the alloy than flat sloping plate.(3) Heat and solute diffusions during solidification of the alloy were obtained. The temperature along the central line of the sloping plate decreased linearly from the casting mouth to the exit. Due to flowing and vibrating, solute diffusion in the melt was strengthened, thus it led to the uniform distributions of solute, which created good conditions for spherical grain formation.(4) The microstructure formation mechanism during dynamical solidification of semisolid alloy was revealed. Three nucleation mechanisms occured:Heterogeneous nucleation on the surface of sloping plate; Eruptive nucleation in the melt with homogenous solute distribution; Nucleus departed from the sloping plate surface under flushing and vibrating, which would cause nucleus multiplication. The growth of non-dendrite microstructure had two patterns:the first was direct globular growth; the second was dendrite growth after the interface was unstable, which was followed by dendrite breaking and spheroidization.(5) The effects of process parameters on solidification microstructures and optimal parameters for preparing semisolid Al-6Si-2Mg alloy were revealed, the sloping plate angle was 45°, the plate length was 400mm, casting temperature was between 660℃and 690℃, the vibrating frequency was between 50Hz and 60Hz, and the sloping plate was designed with sinusoid shape.(6) Microstructure evolution mechanism during remelting process of semisolid alloy was investigated, and remelting process parameters were optimized. the optimal reheating conditions was that the reheating temperature of 610℃and holding time of 60min.(7) The deformation mechanism and mechanical properties of semi-solid alloy were studied. Deformation constitutive equations were established. The deformation process of semi-solid alloy could be divided into two stages:elasticity deformation stage and stable flow deformation stage. the deformation constitutive equations were as follows:(8) Thixo-forging of semi-solid alloy was successfully implemented, and the shape of the product was good, and the surface has no defects, the sizes of the wheel meet the demand of the object. The Vickers-hardness of the product was 100HV after T6 treatment and was improved by 30% compared with conventional casting product of A356 alloy at the same heat treatment. |
PDF Full Text Request